"Quercus agrifolia" by Scott Zona is licensed under CC BY-NC 2.0
By Frank Landis, Conservation Committee Chair & Rare Plants Chair
Time to haul out some math, to make a pitch for saving big trees, although at first this advice seems charmingly quaint, maybe even crazy. The problems du jour are Gold Spotted Oak Borer (Agrillus auroguttatus) and climate change, but this also applies to shothole borers.
There are two arguments for getting rid of big old oaks. One is that they get killed by boring beetles, and so they’re casualties waiting to happen, and not worth saving when they get infested. It’s better to spend scarce money planting seedlings.
The second argument is that seedlings take up carbon faster than do old trees. Therefore, if you want more carbon removal, plant lots of young trees. Old trees are, at best, stores of old carbon. We don’t want them to burn but…maybe cut them down to make houses? Obviously this last doesn’t apply to our local oaks, but it certainly applies to conifers.
I’ll take these in reverse order. Sorry, this is going to involve math, calculating volumes. I know, it’s horrible, take the time to prepare yourself…
…Ready? Sorry for the sarcasm, but I’m trying to defuse any unnecessary cases of math anxiety.
To keep this simple, I’m going to simplify a tree into a cylinder of wood, the trunk. Yes, I know oaks grow into corkscrews, and that they’ve got a lot of biomass in leaves, and even more in roots. Here’s the little secret: in the example I’m going to give, it doesn’t matter, and I’ll get to why at the end.
Anyway, the formula for the volume of a cylinder is πr2h. πr2 is the formula for the area of a circle of radius r, and h is the height of the cylinder (it’s a tree so it stands upright).
Since this cylinder represents a tree, it takes carbon out of the air by growing, and it grows by increasing r (radius) and h (height). Just to keep it simple, I’m going to say that the increase in h is 100r. If the radius goes from 1 to 2, the height goes from 100 to 200. Doesn’t really matter if we’re talking about 1 to 2 centimeters, inches, or feet, so I’ll talk about units.
So, for a basic cylinder of radius 1 unit, its volume πr2h is π*12*100, or 314 cubic units.
If the tree grows to have a radius of 2 units, its volume πr2h grows to π*22*200, or 2,513 cubic units.
If the tree grows to have a radius of 100, its volume πr2h grows to π*1002*1000, or 314,159,265 cubic units.
If the tree grows to have a radius of 101 units, its volume πr2h grows to π*1012*1010, or 323,678,605 cubic units.
Got it? Now here are the two tricky parts. I’m going to introduce two terms: absolute growth rate and relative growth rate.
Growth rates are change over time. Absolute growth rate is simple change over time. So going from a radius of 1 to a radius of 2, the absolute growth rate of our cylinder tree is 2,513-314, or 2,199 cubic units.
Similarly, the absolute growth rate going from radius 100 to 101 is 323,678,605-314,159,265=9,519,339 cubic units.
Makes sense, kind of boring even? Excellent! Let’s go to relative growth rate, which is how much the tree grew relative to its size when it started. This is something investors care about with money.
So, for our tree growing from 1 unit to 2 units, its relative growth rate is 2,199/314, for a relative growth rate of 7, or 700%.
Conversely, for our tree growing from 100 to 101, its relative growth rate is 9,519,339/314,159,265, or 0.03, 3%.
Now, which do you want to have sucking carbon, a small tree of radius 1, or a big tree of radius 100?
If you answered the small tree, you’re wrong, believe it or not, because it’s not the relative growth rate that matters. That 700% return on a per tree basis looks incredibly sexy. Thing is, when you’re trying to maximize the absolute amount of carbon trapped in wood, you have to look at absolute growth rate, not the relative increase in size. The amount of carbon captured by the tree growing to 101 units in radius is over, 4,328 times bigger than that of the sapling growing from 1 to 2 units in radius.
And if you add in all the branches, twigs, leaves, and roots, the difference gets even more stark. That’s why it’s not worth dealing with them to teach this simple lesson. This is about the difference between absolute and relative growth rates, nothing more.
So why do so many planners and politicians think that seedlings are the answer? Well, they’re partially right, we need more trees. The problem is that the foresters advising them are trained to maximize wood production per unit of time, and that’s a relative growth rate question. So, foresters take what they learned farming trees and figured it made sense for capturing carbon: cut the big trees, plant the small trees with their high relative growth rates, and watch them double in size.
Do this wrong and the absolute amount of carbon captured actually goes down.
It’s a matter of geometry, not biology. Absolute growth rates matter, and even a minute amount of growth spread over a big tree adds up to far more carbon captured than a sapling can pull in. This is the carbon argument for why it’s worth saving big trees, as well as planting small ones.
The other argument is that big trees provide ecosystem services that young trees do not. They produce pollen, nectar, fruits, and/or nuts, because they are sexually mature trees. They also provide food for insects that in turn feed other animals, support more fungi on their roots, mosses on their bark, homes for animals, shade for shade-loving plants and ambient cooling, and so forth. Young trees can’t do this by themselves.
But Gold-Spotted Oak Borers do, in fact, kill oaks. Unless you consistently spray them with a specific insecticide, in which case they can survive. Is it worth it? I’d argue yes. San Diego County is short on trees in general, that’s why we’re trying to plant more of them. But where there are already big trees, we really should make an effort to save them. They pull in more carbon they provide more services, and it will take decades for their replacements to grow as large, assuming climate change allows this to happen.
This issue came up because a local nonprofit has, for the last two years, been paying the City of San Diego’s costs to treat the infested coast live oaks (Quercus agrifolia) in Peñasquitos Canyon. It’s not cheap, and they can’t keep doing it indefinitely. They’re doing it, not just to save some old matriarchal oaks, but to keep the beetles from spreading faster throughout the Canyon. They argue that it’s worth saving the old trees, and this is why.
Additionally, the City is also updating their Climate Action Plan, and they’re counting on having more trees to sequester more carbon. And yes, I’m all for planting trees where there are none.
But I also strongly suggest that the City should start paying to spray for GSOB in Peñasquitos Canyon and elsewhere, to try to control the spread of the beetles. And saving one big oak is cheaper than planting hundreds to thousands of seedlings to take up the carbon the old tree now captures. And that, plus all the life the old trees and their woodlands support, is what matters right now.